Signal processing within communication systems having a communication channel, in an effort to improve the quality of signals passing through the communication channel, has been under development for many years. In the past several years, emphasis has moved largely to the domain of digital communication systems that modulate bit streams into an analog signal for transmission over a communication channel. This channel can be a variety of channel types. Many different approaches are employed in the prior art to try to minimize or substantially reduce the effects of interference that may be introduced into a signal that is transmitted across a communication channel. In particular, the prior art approaches that seek to perform cancellation of interference that occupies a small number of signal dimensions in a signal are typically deficient for a number of reasons as is briefly referenced below. One particular type of interference that these prior art schemes seek to minimize is the narrowband interference that is sometimes referred to as ingress interference. Another type of narrowband interference that may be problematic is the interference of impulse/burst noise. Yet another type of interference that may be problematic is within the code division multiple access (CDMA) context when the interference is on a small number of codes.
One of the main methods employed in the prior art to eliminate this interference is the use of a notch filter. This solution is sufficient in some applications, but the notch filter itself oftentimes causes distortion of the desired signal. In the CDMA context, this distortion is called inter-code interference (ICI). Then, another means must oftentimes be included to remove the very ICI that has been introduced by the notch filter. One way to do this is to de-spread the signal. Then, hard decisions are made using the de-spread signal. The hard decisions are then respread and passed through a copy of the notch filter. The difference between input and output of this filter represents an estimate of the distortion introduced by notching out interference and can be used to remove this distortion. In some instances, this process is repeated several times to achieve an adequate result. Making hard decisions involves the possibility of decision errors. Many iterations may be required to achieve convergence to correct decisions, if convergence to an error-free situation occurs at all.
These prior art approaches described above are deficient in that they suffer the effect of error propagation. The decision circuit is prone to make incorrect decisions, requiring many iterations before the process converges, if it ever converges at all.
Further limitations and disadvantages of conventional and traditional systems will become apparent through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.